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    <meta charset="UTF-8">
    <meta name="viewport" content="width, initial-scale=1.0">
    <title>Document</title>
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    <script>
        let mum1, num2 = 0, num3 = 12, str1 = '', str2 = 'nuihao', flag = null;
        console.log(Boolean(num1));  //未定义型转布尔型：false
        console.log(Boolean(num2));  //数值0转布尔型：false
        console.log(Boolean(num3));  //非0数值转布尔型:true
        console.log(Boolean(str1));  //空串转布尔型：false
        console.log(Boolean(str2));  //非空字符转布尔型：true
        console.log(Boolean(flag));  //null转布尔型：false
        //Boolean()函数会将任何非空字符串和非零的数值转化为：true
        //将空字符串、0、NaN、undefined和unll转换为false.
        console.log('--------');
        //未定义转数值
        console.log(Number(num1));
        console.log(parseInt(num1));
        console.log(parseFloat(num1));
        console.log('-----------');
        //字符串转数值 
        console.log(Number(str1));  //空串转数值
        console.log(parseInt(str1));  //空串转数值
        console.log(parseFloat(str1));  //空串转数值
        console.log(Number(str2));  //非空串转数值
        console.log(parseInt(str2));  //非空串转数值
        console.log(parseFloat(str2));  //非空串转数值
        let str4 = '4.34hello';
        console.log(Number(str4));  //
        console.log(parseInt(str4));  //
        console.log(parseFloat(str4));  //
        console.log('--------');
        let isNaN;
        isNaN = Number(str4);
    </script>
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